The supercritical water reaction has proven effective in upgrading heavy oil in the absence of catalyst or hydrogen. Hydrocarbon conversion reactions are known to improve the quality of petroleum-based fuel such as gasoline and diesel.
Supercritical fluids, such as water, are effective in hydrocarbon conversion reactions. Supercritical fluids act as a solvent for the hydrocarbons, dissolving the hydrocarbons in the supercritical fluid. Supercritical fluids are necessary for if hydrocarbons are exposed to temperatures above the critical point of the supercritical fluid, without being mixed with the supercritical fluid, large hydrocarbon molecules tend to condense into larger molecules which are eventually transformed into insoluble solid coke. Supercritical fluid, as a solvent, can prevent such undesired reactions.
While any supercritical fluid that interacts with hydrocarbons can be used, water, due to its environmental benefits and abundance, is the most popular supercritical fluid used and tested in the industry. Supercritical water behaves like an organic solvent for hydrocarbons due to its low dielectric constant. In the hydrocarbon conversion reaction, supercritical water can be a diluent to disperse hydrocarbons.
One of the important steps in reacting hydrocarbons under a supercritical water is mixing the two fluids. The better the mixing, the more likely the formation of coke in the reactor is reduced. Segregated portions of hydrocarbons which are not dissolved in supercritical water can be easily transformed to solid coke in the reactor. Although it is not yet clearly understood, mixing between hydrocarbons and supercritical water occur in stages. First, light hydrocarbons diffuse into the supercritical water by evaporation and form a single phase. Supercritical water having light hydrocarbons dissolved has higher solubility toward hydrocarbons than neat supercritical water. Second, supercritical water begins to penetration into a portion of the large hydrocarbons which are not evaporated in the first stage. While this second stage tends to a uniform phase, it is possible that the phase is not entirely uniform, but includes tiny hydrocarbon droplets dispersed in the supercritical water. As used herein, “dispersed” means to separate uniformly or nearly uniformly throughout the liquid. In the next stage, the asphaltenic portion, which is difficult to dissolve in the supercritical water, is swollen by supercritical water. The swollen asphaltenic portion (the aggregated asphaltene) eventually pops, or breaks, to form small droplets of hydrocarbons which can then be mixed with the supercritical water.
The asphaltenic portion is the most difficult portion of hydrocarbons to mix with supercritical water. Mixing of hydrocarbon feedstock and, in particular, the asphaltenic portion with supercritical water has many unknown variables and is therefore hard to control. Asphaltene is stabilized in the hydrocarbon stream with the aid of other molecules such as resins, aromatics, and saturates. In particular, resins and aromatics act as surfactants that disperse asphaltene in the hydrocarbon matrix. Destruction of resins and aromatics prior to asphaltene conversion results in aggregation of asphaltene which contributes to coke formation. It is important to disperse asphaltene within the supercritical water as thoroughly as possible before entering the reactor, to prevent the intermolecular condensation reactions that will result in coke formation in the reactor. In addition to dispersing the asphaltene, the supercritical water dissolves the resins and aromatics. However, dissolving the resins and aromatics and dispersing the asphaltenes is a balancing act that must be carefully managed to prevent the aggregation of asphaltene. One way to manage the balancing act is to dissolve or extract the resins and aromatics stepwise while keeping asphaltene in well dispersed state.
In order to maximize the effect of supercritical fluid in hydrocarbon conversion reactions, hydrocarbon feedstock must be very well mixed with supercritical fluid before being subjected to reactor which is operated at high temperatures.
Current mixing units includes mixing tees and mechanical mixers, such as ultrasonic wave generators. A mixing tee is a piping unit that has two inlet ports, one outlet port, and an can include an optional port for a thermocouple. In the case of mixing a hydrocarbon and a supercritical fluid, one inlet port is for the hydrocarbon and one inlet port is for the supercritical fluid. While mixing tees provide a measure of mixing, the tee provides a limited interface between the two fluids, which can limit the extent of mixing. Ultrasonic wave generators use highly sophisticated equipment to achieve nearly “complete” mixing. However, the highly sophisticated equipment necessary to produce ultrasonic energy requires large amounts of energy and is subject to electrical failure. The surface of the ultrasonic wave generating equipment can be eroded by the high energy vibration. In addition, the high energy vibration can cause mechanical failures in parts of the process lines that are joined together.
Therefore, there is a need for a mixing unit to facilitate the efficient contacting of heavy oil with supercritical fluid, which does not result in large amounts of coke or substantial increases in operating costs.